Gap opening in the surface electronic structure of graphite induced by adsorption of alkali atoms: Photoemission experiments and density functional calculations

The adsorbate-substrate interaction for Na and K on highly oriented pyrolytic graphite (HOPG) has been investigated by photoemission and by density functional calculations. With increasing coverage a peculiar spectral structure develops below the Fermi level reminiscent of the one observed for $\mathrm{Cu}∕\mathrm{Xe}∕\mathrm{HOPG}$ and formerly interpreted in terms of a Fano resonance. In contrast, we show that this unusual spectral feature reflects a surprising gap opening in the surface electronic structure of graphite owing to charge transfer upon alkali metal adsorption. This finding allows us to put forward a unified picture for the electronic structure changes upon adsorption of alkali metals (Na, K) on HOPG as well as of alkali and alkalilike metals on noble gas spacer layers supported on HOPG.